Crustacean larva raising method and apparatus

ABSTRACT

A crustacean larva raising method includes the steps of providing a tank ( 50 ) to hold larva raising medium, continuously supplying sterilized, filtered larva raising medium to the tank ( 50 ) through outlets ( 64 ) adapted to cause horizontal circulation of the medium and at an outlet flow velocity preventing larva damage, continuously draining the medium through a drain assembly ( 65 ) including a larva screen and maintaining the medium at a selected temperature. Apparatus to carry out the crustacean larva raising method comprises of modular channel sections ( 51, 52 ) of the tank ( 50 ), which are bolted and sealed together, two sub tanks ( 53, 54 ), a UV sterilizer unit ( 55 ) incorporating temperature control means and a submersible pump ( 56 ) to circulate filtered and pre-sterilized medium. A crustacean larva feeding composition and a method for preparing such a composition are also provided.

This invention relates to a crustacean larva raising method andapparatus.

This invention has particular but not exclusive application to larvaraising method and apparatus for use with Thenus spp., and forillustrative purpose reference will be made to such application.However, it is to be understood that this invention and inventiveelements thereof could be used in other applications, such as rocklobster and slipper lobster larvae.

There have been many attempts made to develop larva-rearing strategiesfor commercial species of crustacea. To date these have beenconcentrated on developing strategies for species of rock and slipperlobster larvae. A summary of these processes is given in Table 1.

TABLE 1 No. sur- Tank Species Authors Survival vived size South AfricanKittaka <1% 1 from 100 l rock lobster (1988) 15,000 (Jasus lalandii)Southern rock Kittaka et al. <1% 2 from 100 l lobster (Jasus (1988),16,000 72 l × 4 edwardsii) Illingworth et al. <1% 1 from (1997)  6,000European rock Kittaka and Ikegami <1% 1 from 100 l lobster (Palinurus(1988)  5,000 elephas) Japanese rock Yamakawa et al. <1% 1 from 1 llobster (1989)  1,000 bowls (Panulirus Kittaka and Kimura <1% 2 from 100l japonicus) (1989) 20,000 Slipper lobsters Takahashi and  1.8% 6/330100 l (Ibacus spp.) Saisho (1978) (nistos) Marinovic et al. 15% 6 from 7l (1994)    40

A chronology of larval efforts is given in Table 2.

TABLE 2 Bay Lobster Year species Survival Other Species Survival 1978Slipper lobsters 6 from 40 (Ibacus spp) Takahashi and Saisho 1988 SouthAfrican rock 1 from 15,800 lobster (Jasus Ialandii) Kittaka 1988Southern rock 2 from 16,000 lobster (Jasus edwardsii) Kittaka et al.1988 European rock 1 from 5,000 lobster (Palinurus elephas) Kittaka andIkegami 1989 Japanese rock 1 from >1,000 lobster 1 from 20,000(Panulirus japonicus) Yamakawa et al. Kittaka and Kimura 1994 Slipperlobster 6 from 330 (Ibacus peronii) Marinovic et al. 1995 Successful 1larvae/L larval rearing up to 100 by small vessel (Survival (1 L glass0-80%) bowls) 1997 Southern rock 1 from 6,000 lobster (Jasus edwardsii)Illingworth et al.

The present four major commercial-research rearing tank systems for rockand slipper lobster larvae are, for the Southern rock lobster, (Jasusedwardsii) the systems developed by the Tasmanian Aquaculture andFisheries Institute (TAFI), for J. edwardsii and the Eastern rocklobster, (Jasus verreauxi) the systems developed by the NationalInstitute of Water and Atmospheric Research (NIWA) of New Zealand, forthe Japanese rock lobster, (Panulirus japonicus) the system developed byFisheries Research Institute of Mie (FRIM), the Japan Sea-FarmingAssociation (JSFA) and Research Institute for Science and Technology ofThe Science University of Tokyo, all of Japan. The broad features ofthese systems and the results published are as follows:

1. (TAFI)

Southern rock lobster

10 l water in 30 l tanks

Stocking density of 20 newly hatched larvae per l

No juveniles obtained.

2. Upwelling tank system (NIWA)

Southern rock lobster

Combination of 4×72 l tanks

Stocking density of 26 newly hatched larvae per l

Only one juvenile survived in 1990

3. Zero water movement tank system (FRIM)

Japanese rock lobster

150-180 l in 200 l tanks

Stocking density of 20 newly hatched larvae per l

Less than 1% survival to the juvenile stage (up to 10 juveniles)

4. (JSFA)

Japanese rock lobster

150-180 l in 200 l tanks

Stocking density of 20 newly hatched larvae per l to 1 final stagelarvae per l

Approximately 1% survival to the juvenile stage (up to 100 juveniles)

Thenus spp., commonly known as Moreton Bay bugs, Slipper lobsters andBay lobsters, are found along the entire northern coast of Australiafrom Shark Bay in Western Australia to Coffs Harbour in northern NewSouth Wales (Kailola et al., 1993). There are two Thenus species: Mudbugs (Thenus sp.) and Sand bugs (Thenus orientalis). Mud bugs are brownoverall and have brown stripes on their walking legs, while Sand bugsare speckled overall and have spots on their walking legs. Mud bugsprefer a bottom of fine mud, and are typically trawled from inshorecoastal waters of 10 to 30 meters depth. Sand bugs tend to prefersediments with a larger, coarser particle size, and are usually trawledfrom a depth of 30 to 60 meters in the coastal shelf and offshore areas.

Currently, commercial aquaculture of Moreton Bay bugs is not beingcarried out anywhere in the world. The major hurdle in commercialisationis the difficulty in maintaining the bugs through the larval stages.Like other slipper or rock lobster species, Moreton Bay bugs have a verycharacteristic flattened larval stage called the phyllosoma. Theycirculate in the plankton, rising and falling in the water column, andthis makes it difficult to adjust the culture environment.

Recently however, a comprehensive study of the culture conditions ofMoreton Bay bug phyllosomas was undertaken, suggesting high potentialfor commercial aquaculture of these species (Mikami, 1995). Phyllosomasof Moreton Bay bugs pass through four larval stages in 25 to 35 days,with a high level of survival on a small scale and take one year toachieve commercial size (250 g).

Following the study by Mikami (1995), further research has beenundertaken by the present applicant over the past five years. The majoraim of this study is the commercialisation of Moreton Bay bugaquaculture from the small, experimental scale. To date, the single mostimportant issue has been to solve the technical aspects of Moreton Baybug larval rearing.

In one aspect this invention resides in a crustacean larva raisingmethod including the steps of:

providing a tank adapted to hold larva raising medium to a depth of atleast 10 cm;

continuously supplying substantially sterilized, filtered larva raisingmedium to said tank through a plurality of outlets disposed about thetank and adapted to cause horizontal circulation of said medium andhaving an outlet flow velocity selected to prevent larva damage;

continuously draining said medium through a drain assembly including alarva screen having a flow velocity of said medium therethrough selectedto prevent damage to larvae, and

maintaining said medium at a temperature selected to accommodate thelarva species to be raised.

In a further aspect, this invention resides broadly in crustacean larvaraising apparatus including:

a supply of substantially sterilized, filtered larva raising medium;

a tank adapted to hold said larva raising medium to a depth of at least10 cm;

a plurality of outlets connected to said supply and adapted to deliverand cause horizontal circulation of said medium in said tank;

drain means having a larva screen and configured to maintain a selectedlevel in said tank, and

temperature control means for said medium.

The larval rearing tank may be round or oval in horizontal cross sectionsuch that a continuous one-way circulation may be maintained.Alternatively, the larval rearing tank may comprise an annular tank. Asa yet further alternative, the larval rearing tank may comprise anannular raceway having straight portions closed by end portions.Preferably, the raceway comprises a modular construction of curved andstraight portions, whereby the linear dimensions and thus holdingcapacity may be selected. For example, the modular components may bemoulded in plastics material and be adapted to be bolted up in assemblyto form the raceway. The modular components may be provided withpreformed joint sealing, or may in the alternative be sealed with an insitu cast sealing such as curable silicon or other sealant.

The tank depth is preferably less than one meter. Preferably, the waterdepth is maintained at about 10 to 20 cm. This relatively shallow depthwill allow increasing feeding frequency of larvae. In the case of thepreferred annular and or modular raceway construction, the section ofthe raceway may be for example 30 cm deep. Whilst the width of thesection may be of any suitable dimension determined at least in part bythe arrangement of the rearing media outlets, it is preferred that thisdimension also be in the region of 30 cm.

For phyllosomas of Moreton bay bugs, a typical stocking density of about40 newly hatched larvae per l is used, gradually reduced to about 10-15final stage larvae per l.

In order to increase floor area densities in industrial situations thetanks may be arrayed in stacks.

The medium will be selected according to the species to be raised. Ingeneral the medium will be seawater or synthetic seawater of compositionselected to match the natural environmental medium in which the organismexists in the wild.

The water outlets may comprise a plurality of nozzles. The nozzles maybe of a number selected to encourage the continuous one way circulationwith consistent flow about the circuit. The number of nozzles andcapacity of tank can be used to adjust the volumetric flow rate.

The flow velocity of the nozzles may be any flow velocity selected tomaintain circulation in the tank whilst avoiding shearing injury to thelarvae. The flow velocity is preferably maintained in the region ofbelow 4 to 6 m per minute at least for early larval stages such as theat the 1 st phyllosoma stage of Moreton bay bugs. Preferably the flowrate is the minimum flow rate consistent with maintaining circulation ofthe medium in the tank.

The outlets may be located at the bottom of the tank or the top of thetank. For example, the outlets may be associated with a linear or ringmains manifold located at the bottom of the tank or at any position upthe walls of the tank including above the medium level. There may beprovided a single manifold or a plurality of manifolds. In oneembodiment of the invention associated with the preferred modularraceway tank, the outlets are tubes with 4 mm nozzles extended from 19mm polyethylene manifolds disposed on the upper portion of the inner andouter walls of the raceway and installed after assembly of the modularstructure. The outlets extend down to the bottom of the tank and thenozzles are aimed in the direction of desired circulation and preferablydirected somewhat inwardly away from the walls.

The medium may be supplied by a continuous one way system or may utilizesome recirculation. For example, in a one way supply, the medium such asseawater may be filtered from a natural supply through a 1 μm filter andpreferably a 0.5 μm filter in a header tank.

The drain means is preferably provided with a mesh size of about 1 mm.The flow rate across the mesh may be determined in each caseempirically. However, it is preferred that the flow rate per unit areabe much less than the inflow velocity and accordingly the surface are ofthe mesh is preferably maximized. The drain means may be used tomaintain the level of medium in the tank. To this end the drain meansmay comprise a surface drain set to the desired level or 10-20 cm.Alternatively, the drain means may be located at any level in the mediumcolumn, whereupon the level may be medium supply/drain rate controlled.In cases where the drain controls the level, this may be provided bymeans of a meshed drain inlet of enlarged area relative to a standpipetaking the medium to waste or recirculation, which standpipe may beadjustable in length.

The tank may be provided with a cover or other means selected to occludelight from the tank at selected intervals. Light should be excluded fromthe tank in the daylight hours to maximise feeding frequency of thelarvae. To this end it is preferred that the tank be formed of an opaquematerial.

Contamination by bacteria, protozoa or fungi is a serious problem forphyllosoma rearing. The major sources of contamination for larvalrearing are incoming water, food, the air, human handling and thestarter culture (eggs, water and newly hatched larvae from the hatchingtank). Larval rearing water should be kept free from any organisms.Accordingly, after filtration the filtered seawater may then besterilized by any suitable means such as UV sterilization, submicronfiltration, chlorination, acidification or ozonisation. For example, thefiltered seawater may be exposed to UV radiation from an arc or othersource at above about 1 l/hour/Watt level to minimise bacteria.

Alternatively, chlorine at about 10 ppm may be maintained in incubationwith the medium for about 12 hours, preferably without aeration,followed by the addition of sufficient sodium thiosulfate to neutralisethe chlorine.

The temperature of the filtered, sterilized supply may be maintained inthe desired range by any suitable means such as heaters and/or chillerswith appropriate thermostats.

In the alternative to the one way system, there may be provided asemirecirculation system including the larval rearing tank and two ormore sub tanks. Preferably, two sub tanks are used. The sub tanks areeach of at least the same capacity as the larval rearing tank. In thisembodiment, the sub-tank containing filtered, sterilized water may becirculated into the filled larval rearing tank for about 24 hoursresidence using a submersible pump. After 24 hours, the pump may betransferred to the other sub-tank, with the water controlled at aboutthe same temperature, preferably within about ±0.5° C. Water may then becirculated into the larval rearing tank again. Preferably, the flow rateis the same as the one way flow-through system. While water in onesub-tank is being used, the other sub-tank may be emptied and dried.

In the case of semirecirculation, the rearing water may be sterilizedby, for example, 10% chlorine for a period of 12 hours followed byneutralisation with 10% sodium thiosulphate. Preferably, the rearingwater is tested such as by Palintest® (DPD No 1) before introducing intothe rearing system to make sure no chlorine remains.

In the case of phyllosomas of Moreton bay bugs, the temperature range ofthe medium is preferably between 26-27° C. Phyllosomas can be reared attemperature ranges between 24-30° C., but temperatures lower than 26° C.will result in a slower growth rate, and those higher than 28° C. willincrease the risk of unsuccessful moulting, cannibalism and disease.When larvae are transferred to the rearing tank, it is preferable tokeep the temperature of the larval rearing system at substantially thesame temperature ±0.5° C. as the source of the larva such as a hatchingtank. If rearing water temperature has to be changed, temperaturevariation is preferably kept within 1 degree per hour.

The salinity of the medium may vary according to species. In the case ofphyllosomas of Moreton bay bugs, the salinity may be maintained in therange of between 25-40 ppt and preferably between 34-36 ppt. Phyllosomasare extremely intolerant to sudden changes in salinity, so salinitychange should be kept within ±1 ppt per day.

Throughout all phyllosoma stages, phyllosomas show strong photopositivereaction. To avoid congregation of larvae at the surface during thedaytime, the rearing system may be covered such as by a black plasticsheet.

The level of pH may be kept at between 7-9, and preferably between8.2-8.5 being the natural seawater pH level.

Strong aeration damages larvae, so it is preferred to avoid usingaeration in the rearing tank. The oxygen level of the rearing water ispreferably kept at more than 7 ppm, at 26-27° C. Larval oxygenconsumption is very low, so the circulation of rearing water with alarge surface area is generally adequate for supplying larval oxygendemand without aeration, with control of stocking density.

Under the flow-through system, the preferred maximum rearing densitiesof phyllosomas are:

40 first instar larvae per l

25 second instar larvae per l

10 third instar larvae per l and

5 fourth instar larvae per l.

Rearing density levels higher than this may result in a high level ofcannibalism at the time of moult. Pre-moult/post-moult larvae are eatenby intermoult larvae.

The phyllosomas are preferably fed a controlled diet. The maximumphyllosoma growth and survival has been determined to be obtained by theuse of chopped, fresh, live mollusc flesh, preferably live pipis (Donaxspp.). Using frozen food will result in a slower growth rate than freshfood. Brine shrimp (Atemia spp.) can also be used, but only for 1stinstar phyllosomas.

The use of live pipis occasionally causes a high level of mortalities atthe time of moulting. This is called moult-death syndrome (MDS). Thecause of MDS amongst other species is still unclear, but in the case ofThenus, MDS is related to seasonal variances in food quality. To obtaina standard quality of food, enrichment of bivalves is preferred.

Use the green micro-algae Nannochloropsis spp. or other micro-algaeand/or diatom species such as Isochrsis spp., Chaetoceros spp. andPavlova spp. for enrichment has proven useful. Enrichment may compriseculturing pipis at 25-28° C. with algae water at a cell density ofpreferably greater than 20×10⁷. For example, there may be usedapproximately 1 kg of pipis (wet weight with shell) per 40 l of algaewater. Preferably, the water is replaced every 12 hours. The enrichmentprocess may be conducted for at least 24 hours and preferably 48 hours.The level of ammonia in the algae water should be maintained below 1ppm. Flesh content of pipis (gut, gonad, gill and mantle) isapproximately 20% of total weight.

Alternatively to the preferred algae, dried commercial species may beused such as Marine Sigma (Nisshin Science), Marine Growth (NisshinScience), and Algamac-2000 (Bio Marine). The number of cells of thesecommercial products should be kept at >20 million per ml.

Food preparation may be by any suitable means. In the case of thepreferred pipis, the flesh may be chopped roughly followed by washingthrough at least two grades of mesh, such as 0.5 to 2.0 mm for a firstwash and thereafter <0.5 mm. The large mesh size is preferably variedaccording to larval stage. For example, there may be used 1.0 mm for thefirst instar, 1.5 mm for the 2nd instar and 2.0 mm for the 3rd and 4thinstars. The pieces of chopped flesh retained between the large andsmall mesh sizes may be set aside. The pieces of flesh retained in thelarge size mesh may be chopped again, repeating the above process.

Food must be sterilised before feeding in order to avoid bacterialcontamination. For example, the flesh may be washed in UV sterilisedseawater carefully, and then incubated in 0.1% chlorine seawatersolution for a period of 30 minutes or more. Then wash the foodparticles by UV sterilised seawater again on the small mesh beforefeeding to larvae.

Prepared food materials with seawater may be distributed equally in therearing water using for example a pipette. Food particles will sink tothe bottom of the rearing tank. Food particles remaining in the rearingtank after feeding should be cleaned out before adding the next lot offood. The feeding level changes depending on growth stage and intermoultstage. The level of feeding should be adjusted by taking note of howmuch food remains from the previous feed.

Phyllosomas start eating from the night of hatching. To obtain synchronyof larval moult, it is preferred to not feed on the morning of Day 1. Asthe phyllosomas start eating more, so preferably adjust the feedinglevel depending on level of remaining food. Feed preferably twice a dayin the early morning and late evening. At Day 5-6, the phyllosomas startpreparing to moult, so the feeding level may be decreased from theevening of Day 5.

First instar phyllosomas moult to the second instar in the earlymorning, so the feeding level in the morning may be minimised, with morein the late evening. On days 7-9, feeding twice a day is stillacceptable but towards Day 9 phyllosomas start to eat more. Monitoringthe level of remaining food regularly is preferred to avoid starving thephyllosomas, feeding 3 times a day if necessary. On days 9-10, feedinglevels will still be high, even before moulting. It is preferred to makesure enough food is available through the nights to avoid cannibalism inthe morning.

Larvae usually moult to the third instar in the early morning (4-5 am),and therefore it is preferred to make sure enough food is availablebefore and during the moulting stage. An extra feeding before moultingis desirable, if there is no food remaining in the tank. Post-moultstage larvae will not eat food for 3-6 hours, and therefore the morningfeed should be minimised, with a higher level in the afternoon. From Day12-16, larvae may be fed 3 times a day, preferably every 8 hours suchthat food is always available. Starvation of phyllosomas will cause ahigh level of cannibalism when third instar phyllosomas moult to fourthinstar phyllosomas.

Fourth instar phyllosomas (pay 15-27) may be fed over Days 15-17 atthree times a day, preferably making sure food is always available. Atdays 18-21, the feeding level of phyllosomas is now at its peak. Larvaemay be fed three times a day or more, preferably without delaying any ofthe three feedings for more than 2 hours. From day 21-30, phyllosomasstart to metamorphose to the nisto stage, and therefore the feedinglevel should be decreased with the decreasing number of fourth instarphyllosomas. When phyllosomas are not eating food around day 25-26,feeding can be reduced to only twice a day.

Under optimal rearing conditions (physical and nutritional), intermoultperiods of phyllosomas are preferably synchronised. The timing of thesemoultings depends on the rearing conditions (temperature, foodcondition, stock density and so on), and therefore it is preferable tomaintain optimal rearing conditions throughout all larval stages.

Larvae can be reared using only one rearing tank in accordance with thepresent invention, with no tank exchange required. When fourth instarphyllosomas metamorphose to the nisto, pre-metamorphosis phyllosomasshould be transferred to the nisto tank. Metamorphosis always occurs inthe late evening just after sunset. Pre-metamorphosis phyllosomas can beidentified by changes in the external morphology: the appearance of “W”shaped gaps at the basement of the antenna (these become the eyesockets); small dots on the carapace; and the changing of body colour towhite. Pre-metamorphosis phyllosomas should be transferred with seawaterto the nisto tank.

To avoid bacterial contamination, human contact with larval rearingwater must be avoided. For example, it is preferred to wash hands withan anti-bacterial soap before the treatment for larvae. Plasticinstruments may be kept, for example, in a 0.01% chlorine water bathwhen they are not being used. Preferably, change the water completelyevery 3-4 days. Glass instruments may be washed carefully with freshwater and keep on a shelf when dry.

Phyllosomas can start eating immediately after hatching, but thisdepends on yolk retention of larvae and temperature. In general,phyllosomas start eating 6-12 hours after hatching, but can survive forup to 72 hours without food. Starvation time of up to 48 hours at 27° C.will have no influence on survival and moulting. The 50% level of Pointof No Return (PNR₅₀) is generally 72 hours after hatching, but this willchange depending on yolk retention of larvae. Delaying the initialfeeding will prolong the duration of the first instar. After moulting tothe second instar, the initial starvation will have no further influenceon growth.

Phyllosomas eat less food at pre- and post-moulting stages (±12 hours ofmoulting) than during the middle of the intermoult periods. In themiddle of the intermoult periods, phyllosomas eat constantly, day andnight. Although phyllosomas have a strong capacity for starvation andcan survive for more than 72 hours without food, long-term starvationand lower feeding levels will result in an increased risk of MDS at thetime of moulting.

Phyllosomas are not passive feeders; they approach and attack prey. Thephyllosomas attack (pick up) the food using pereiopods, and pass to themouthparts, located at the central part of the carapace (ventral side).The mouthparts comprise the labrum, pared paragnaths, mandibles and 1stmaxilliped. The labrum and paragnaths cover the top of the mandibles.The food particles are pushed onto the paragnaths by the 1st maxilliped,and cut roughly into small food masses. Then the mandibles, which have ascissor-like structure on the anterior-tip, break down the food intoeven smaller pieces. Therefore phyllosomas can only eat soft food masseswith a high water content.

After phyllosomas ingest food materials into the gut system, the colourof the midgut gland changes from transparent to white, due to theappearance of lipid rich globules within the cytoplasm of the midgutgland cells. Only a portion of the ingested food materials goes into themidgut gland area, where the main part of digestion occurs. The majorityof food materials pass through the midgut tubule and are excluded viathe anus, 5 to 10 minutes after eating. Faeces of phyllosomas arelipid-rich pseudo-faeces.

Phyllosomas are plankton, and usually swim in the same direction as thewater current. However, phyllosomas are also strongly photopositive andcan swim across a water current of 10-15 m per minute towards a lightsource. In the hatchery, phyllosomas congregate at the spot where lightintensity is the highest during the daytime, but spread themselvesevenly in the water column during the night-time. Phyllosomas show astrong photopositive phototaxis even under illuminance levels of 0.5μEm⁻²sec⁻¹. Phyllosomas can also swim to the bottom of the tank and pickup food materials against a water current of 10-15 m per minute. Whenphyllosomas are healthy, they swim with rotation of their bodies.

Moulting usually only occurs in the early morning around sunrise. Thepre-moult stage (where the internal chemical composition of phyllosomaschange) starts 2-3 hours before the actual moulting. The pre-moult stagelarvae can be identified by a change in body colour (transparent towhite-pink) and swelling of the carapace. Post-moult phyllosomas arevery soft and fragile for 2-3 hours. Movements of post-moult stagephyllosomas will depend on the water current. Cannibalism occurs only atthe time of moulting, when intermoult larvae eat post-moult andpre-moult stage phyllosomas. Post-moult phyllosomas start to eat food2-4 hours after moulting.

Metamorphosis only occurs in the late evening around sunset, with theprocess lasting only 10-20 minutes. Pre-metamorphosis phyllosomas can bedistinguished by their external morphology: small dots on the middle ofthe carapace (these become the top edge of carapace after metamorphosis)and “W” shape gaps at the basement of antenna (these become the eyesockets). The entire body of pre-metamorphosis phyllosomas is thick andtends to be bright-white.

Cannibalism of phyllosomas can be observed only at the time of moulting,where intermoult stage phyllosomas eat pre- and post-moult stagephyllosomas. However, if nutritional requirements of phyllosomas aresatisfactory throughout the intermoult period, the level of cannibalismshould be minimised.

The causes of phyllosoma diseases can be divided into bacterial, fungal,nutritional, viral and environmental or stress origins. Contamination bybacteria is the major problem in rearing phyllosomas. The most commonsources of bacterial contaminations are starter culture (eggs), incomingwater and foods. Sterilisation of the seawater by physical methods, suchas filtration, UV, and/or chemical methods, such as chlorination andozonisation, is effective in preventing diseases of bacterial origin.There are a number of critical diseases originating from bacteria.

Toward the end of larval rearing (amongst fourth instar phyllosomas),Vibrio infection will result in gut blockage. The symptom isaccumulation of food material in the midgut tubule (constipation).Larvae will die after 6-12 hours. The midgut path is infected by Vibrio,and gut material is no longer excluded from the midgut tubule. Thisdisease is not contagious. An accumulation of bacteria on the bottomsurface of the rearing tank is thought to cause this disease. Dailymaintenance, particularly of the bottom surface, is an important factorin prevention. Alternatively, exchanging the rearing tank can reducemortalities from this disease.

The pared antenna glands are an excretory organ located at the base ofthe antenna. The antenna glands are surrounded by a single cell layercalled a bladder, where ammonia is selectively transported from thehemolymph. The bladder is connected to the opening at the surface of thecarapace, and ammonia is excreted via the opening. The diameter of thisopening is less than 5 μm. Because of a high level of ammonia around theopening of antenna glands, filamentous bacteria can grow easily andshade the opening of the antenna gland, causing necrosis at a celllayer. Healthy antenna glands are transparent, but their colour willchange to brown/black after necrosis. If both antenna glands areinfected, larvae will die after 24-48 hours. Streptomycin at 10 ppm canprevent bacterial growth, but there is no cure for necrosis of theantenna gland. Daily cleaning of the larval tank system is an importantmeans of reducing mortalities caused by this type of disease.

Filamentous (Leucothrix sp) bacterial infection can be observed on thesurface of the exoskeleton. Poor water quality management is the cause.Though streptomycin sulphate at 10 ppm is an effective preventativemeasure, the continual use of antibiotics should be avoided.

Fungal origin diseases may include infections from marine fungi commonlyfound in seawater, and occasionally grow on the surface of the larvalexoskeleton, particularly on the exopodal setae of periopods. Marinefungi start to grow 3-4 days after hatching and moulting. Fungi growingon the exopodal setae sticks will attract food particles, and as aresult larvae will be “glued” together. These larvae will not dieimmediately, but feeding and swimming will be destructive, causing highmortality two or three days after the appearance of the fungi. Formalinat 20 ppm twice a day can prevent marine fungi growth, but will not beeffective against fungi already on the surface of larval exoskeleton.

Nutrition is an important factor for not only obtaining optimal growth,but also for prevention of any kind of disease. Phyllosomas should havesome degree of resistance against diseases of bacterial origin if theirnutritional requirements have been adequately met. Moult Death Syndrome(MDS) is a catastrophic syndrome, observed at the time of moulting.Larvae simply stop moulting in the middle of the process and die. Thissyndrome cannot be predicted until the time of moulting. Duringintermoult periods, larval survival and activity are always high, andintermoult periods are usually synchronised. Seasonal variation in thenutritional content of natural foods (bivalves, pipis) is considered tobe the major cause of MDS. During early spring (September) to mid-summer(December), a high level of MDS is observed with the feeding ofnon-enriched pipis. MDS levels are much lower between mid-summer(January) and autumn (June).

Enrichment of pipis (improvement of nutritional value) as describedabove is an effective way to prevent this syndrome. However,occasionally MDS will still occur even when phyllosomas are fed onenriched pipis. Obviously, some other factors, such as high density(less individual feeding) and inconsistent environmental conditions(inconsistent nutritional uptake) are inter-related in some way withMDS.

Diseases originating from environmental (physical and chemical) stressare not contagious. Environmental stress can manifest as white spots,with particles in the hemolymph clumping together. A high level ofchemical contamination (chlorine, formalin etc) and physical stress(high stock density, freshwater drops from the cover) is the likelycause. Phyllosomas will die within 24 hours of the appearance of thesewhite spots. There is no treatment.

POST LARVAL (NISTO) REARING

After metamorphosis to the nisto stage, nistos are reared in a nistotank. The exoskeleton of nistos is transparent and not calcified. Thecolour of nistos changes from transparent-white to orange in colour dueto the development of pigmentation under the exoskeleton. Water qualityin the nisto rearing tank is the same as that of phyllosoma rearing.Nistos can be reared at a high density (>100 nistos per l). During thenisto stage, feeding is not required. The duration of the nisto stage isapproximately 7 days, with the temperature kept at 26-27° C. To avoidcannibalism, separate pre-metamorphosis phyllosomas from otherintermoult phyllosomas. The same design as the larval rearing tank maybe used for nistos, such as a one-way circular tank. The water should betreated in the metamorphosis tank in the same manner as the larvalrearing tank. No feeding is required. No aeration is required.

JUVENILE CULTURE

After 7 days, nistos moult to the juvenile stage. Moulting to juvenilesalways occurs during the night. The exoskeletons of juveniles arecalcified and pigmented. Newly moulted juveniles should be collectedfrom the nisto tank the next morning and transferred to the juvenileholding tank.

Juveniles are nocturnal. Preferably, feed only once a day in the eveningand clean the remaining food and faeces out the next morning. Adjustfeeding levels according to the amount of remaining food. The food sizefor first instar juveniles is similar size to that of second or thirdinstar phyllosomas. Chopped flesh of enriched pipis is suitable for upto at least fourth instar juveniles, then non-enriched pipis, squids,scallops and mussels can be used. The optimal temperature for culturingjuveniles is 26-27° C.

BROODSTOCK

After catching, live berried females may be stored in a tank within thevessel, preferably equipped with gentle aeration and/or water exchange.Feeding is not required. Environmental conditions, particularly watertemperature and salinity, should be kept constant during the storage ofthe live animals.

Berried females should then be transported with water. Although berriedfemales can survive without water for around 30 hours, long-term airexposure is physically stressful and occasionally berried females willscrape the eggs from their abdomen several days after delivery to thehatchery.

Small quantities of berried females (<6) can be transported in a plasticbag packed with water (10-20 l) plus pure oxygen. The plastic bag isthen placed in an esky, and sent to the hatchery. Berried females andtheir eggs can travel between 24-36 hours in the bags without illeffect.

For a larger quantity of berried females (>6), a fish transporter isrecommended. More than 1 l of seawater per animal is an appropriatevolume. During transport, pure air or oxygen should be supplied at morethan 2 l per minute. Up to 24 hours of travel by this method will notadversely affect the berried females or the eggs.

The berried females are preferably kept in a holding tank with aseawater exchange of more than 100% per hour with aeration (>2 l air permin). A minimum of 30 l of seawater per animal is required. The seawateris preferably sterilized by UV before use. The tank is preferablycovered such as with a black plastic sheet. The temperature of the waterin the holding tank is preferably kept between 20-28° C., with it beingparticularly preferred to maintain 26-27° C. Daily temperature variationshould no be greater than 1° C.

The females may be fed once a day in the evening, and preferably withcleaning the remaining food out the next morning. Preferably, the foodis selected from the flesh of molluscs, such as pipis, squids andmussels. The food is preferably sterilised by 0.1% chlorine solution forat least half an hour, and then carefully washed by UV sterilisedseawater before feeding.

BROODSTOCK HOLDING SYSTEM

The broodstock holding system may comprise a round or square tank with acapacity of more than 200 l. There is preferably provided a black coversheet on the top of the holding tank to minimise stress of broodstock.Gentle aeration may be provided, typically at about >2 l air per minute.Seawater may be supplied from one end of the tank, preferably from thebottom of tank, and discharged from the other end, preferably from thetop of tank.

The water supply is preferably 1-5 μm filtered and UV sterilised water,with a minimum of 30 l seawater per animal. An exchange ratio of >100%per hour is preferred, at a temperature between 20-28° C. and preferably26-27° C.

Females are preferably fed once daily in the evening with food clean outthe next morning by such as by siphoning, and scraping the bottom with asponge. It is preferred to change the tank to new clean tank every 10-15days.

HATCHING SYSTEM

The hatching system may comprise a round or square tank, typicallyhaving a capacity of 100-200 l. Seawater may be supplied from one end ofthe tank, preferably from bottom. Water may be discharged through a 500μm mesh to prevent escape of larvae. Preferably, there may be a blackcover sheet on the top of the holding tank, with a 15-20 cm opening. Itmay be desirable to provide slow aeration of approximately 2 l perminute around the outlet. The water supply should be 0.5-1 μm filteredand UV sterilised water. Preferably, an exchange ratio of approximately100% per hour is used, with temperature preferably kept at 26-27° C.

The hatching tank should be prepared in the afternoon, and the femalestransferred in the late afternoon. Feeding is not required in thehatching tank. The hatching tank should be sterilised by 0.1% chlorinefor a period of 6 hours after the harvesting of larvae. If larvae havenot hatched out by the next morning, females should be returned to theholding tank, and another hatching tank should be set up.

When the embryos become visibly amber-brown in colour, the individualberried female may be transferred to a 100-200 l hatching tank. Thehatching tank should be prepared in the afternoon, and the femalestransferred in the late afternoon, prior to larvae hatching. Seawatermay be supplied, previously filtered and sterilised by UV, at a rate ofapproximately 100% exchange per hour, exiting from a tank through a 500μm mesh to prevent escape of larvae. Preferably, slow aeration issupplied at approximately 2 l per minute around the outlet.

Hatching always occurs at around sunrise. When the eggs hatch, thefemale flicks her tail several times, and larvae are scattered into thewater. This lasts about 10 to 20 minutes. Hatching occurs only in themorning, and occasionally spreads over two to three mornings. Tominimise stress of the female, lighting should be avoided.

After hatching, larvae are very soft and fragile, and lack swimmingability and therefore strong aeration should be avoided. Theexoskeletons of the larvae become hard, and they start swimming toward alight source 20-30 mins after hatching. Harvesting of larvae is possiblewhen they congregate at the surface of the water (light source).

Larvae can only be transferred with water. A glass beaker or glass bowlis an appropriate vessel for harvesting.

In order that this invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings and examples which-illustrate a preferred embodiment of theinvention and wherein:

FIG. 1 is a side elevation of semirecirculation apparatus in accordancewith the present invention;

FIG. 2 is a plan view of the apparatus of FIG. 1;

FIG. 3 is a detail plan view of a larval raising tank suitable for usein conjunction with the apparatus of FIGS. 1 and 2;

FIG. 4 is an elevation of the tank of FIG. 3;

FIG. 5 is a plan view of a flow through apparatus in accordance with thepresent invention;

FIG. 6 is a not-to-scale elevation of the tank detail of FIG. 5;

FIG. 7 is an alternative embodiment of a semirecirculation system tothat illustrate FIG. 1;

FIG. 8 is an alternative embodiment of a flow through system to thatillustrated in FIG. 5; and

FIG. 9 is an illustration of a fully recirculated larval raising systemin accordance with the present invention.

In the Figures, like elements are given like numerals where appropriate.

EXAMPLE

BROODSTOCK HOLDING SYSTEM

Seawater is continuously supplied to a 1 μm filter and thence to a UVsterilizer. The water supply is set at 30 l seawater per animal with anexchange ratio of 100% per hour. The temperature is maintained between26-27° C. The sterilized water is supplied to a round tank with acapacity of 200 l. A black plastic cover sheet is provided on the top ofthe holding tank to minimise stress of broodstock. The tank is gentlyaerated at >2 l air per minute. The water is supplied continuously toone end of the bottom of the tank and discharged from the other end atthe top of tank.

The berried females are introduced to the tank and fed once a day in theevening. The tank is cleaned of remaining food the next morning. Thefood is pipis sterilised by 0.1% chlorine solution for at least half anhour, and then carefully washed by UV-sterilised seawater beforefeeding.

When the embryos become visibly amber-brown in colour, the individualberried female is transported to the hatching tank.

HATCHING SYSTEM

Water is filtered and sterilized as before and continuously supplied atan exchange ratio of approximately 100% per hour, the temperature beingmaintained at 26-27° C. the water is supplied to a round tank withcapacity of 200 l. The treated water is supplied from one end of thetank from bottom. Water is discharged through a 500 μm mesh to preventescape of larvae. A black cover sheet covers the tank and has a 15-20 cmopening. Slow aeration at 2 l per minute is supplied around the outlet.

The hatching tank is prepared in the afternoon, and the femalestransferred in the late afternoon. Feeding is not required in thehatching tank. The hatching tank is sterilised by 0.1% chlorine for aperiod of 6 hours after the harvesting of larvae. If larvae have nothatched out by the next morning, females are returned to the holdingtank, and another hatching tank is set up.

Hatching always occurs at around sunrise. When the eggs hatch, thefemale flicks her tail several times, and larvae are scattered into thewater. This lasts about 10 to 20 minutes. Hatching occurs only in themorning, and occasionally spreads over two to three mornings. Tominimise stress of the female, lighting is avoided.

After hatching, larvae are very soft and fragile, and lack swimmingability, and therefore strong aeration is avoided. The exoskeletons ofthe larvae become hard, and they start swimming toward a light source20-30 mins after hatching. Harvesting of larvae is possible when theycongregate at the surface of the water towards the light source.

Larvae can only be transferred with water, and a glass bowl accordinglywas used to transfer the hatched larvae.

LARVAL REARING

Referring to the FIGS. 1 and 2, a supporting base comprising piers 10supports a tank frame 12 including tank bearing portions 13 adapted toprovide a desirable working height. A rearing tank 14 is supported onthe bearing portions, and is provided with a black plastic cover 15 foruse in the daytime.

A series of three sub-tanks 16 are provided, the sub-tanks beingsupplied by a one-micron filtered seawater supply (not shown). Thecapacity of each sub-tank 16 is the same as the larval rearing tank 14.

A submersible pump and filter assembly 17 is adapted to be selectivelymoved from one sub-tank 16 to the others. Each sub-tank is provided witha thermostat controlled heater 18. Each sub-tank 16 has a cocked drain20 communicating with a waste drain 21.

The submersible pump and filter assembly 17 has an outlet connected to aflexible pipe 22 that supplies water to a UV sterilizer 23. The UVsterilizer 23 supplies sterilized water to the tank 14 via hose 24. Thewater depth in the tank 14 is set at 15 cm by the height of open-toppedstandpipes 25 extending up through the floor of the tank 14 andcommunicating with a drain manifold 26 which returns the water to thesub-tanks 16 via drain 27. The tank 14 has a mesh assembly 31surrounding the standpipes 25 and extending from the tank floor to abovethe stand pipes 25.

Referring to FIGS. 3 and 4, the water supply hose 24 supplies water to amanifold 32 adapted to even out distribution to an inflow nozzle annulus33 disposed about the peripheral inner surface of the bottom of the tank14. A plurality of nozzles 34 is disposed on the inflow nozzle annulus33 the nozzles all being directed so as to induce a one-way endlesscirculation in the tank 14. The nozzle flow velocity is controlled to 5m per minute at the 1st phyllosoma stage, and gradually increased to15-20 m per minute at the 4th stage.

In use, water in one sub-tank 16 is circulated into the larval rearingtank 14 for 24 hours, using the submersible pump 17. After 24 hours, thepump 17 is transferred to the other sub-tank 16, with the watercontrolled at the same temperature (±0.5° C.). Water is then circulatedinto the larval rearing tank 14 again. The flow rate is the same as theone way flow-through system.

While water in one sub-tank is being used, empty and dry the othersub-tank. The rearing water supplied to the sub-tanks 16 is sterilizedby 10% chlorine for a period of 12 hours, and then neutralised with 10%sodium thiosulphate. Chlorine neutralization is confirmed by Palintest®(DPD No 1) before introducing into the rearing system.

In the embodiment illustrated in FIGS. 5 and 6, a siphon 35communicating with an external overflow tank 36 maintains the waterlevel. Water in this embodiment is supplied to the UV sterilizer 23 froma raw water inlet 37 via a filter to a header tank 41. The header tank41 has a heater and thermostat assembly 42. The header tank feeds the UVsterilizer 23 via supply pipe 43.

In use, the water at the inlet 37 is filtered through the 0.5 μm filter40 and supplied to the header tank 41. After supply to the header tank42 the filtered, temperature controlled (26-27° C.) water is passed tothe sterilizer 23 where it is subjected to 10 l/hour/Watt UV radiation.The salinity range is maintained between 34-36 ppt with salinity changebeing kept within ±1 ppt per day. To avoid congregation of larvae at thesurface during the daytime, the rearing system is covered by a blackplastic sheet. The level of pH is kept at between 8.2-8.5 (naturalseawater pH level). The oxygen level of the rearing water was maintainedat more than 7 ppm by the circulation of rearing water without aeration.

Rearing density

Under the flow-through system of FIGS. 5 and 6, the maximum rearingdensities of phyllosomas are:

40 first instar larvae per l

25 second instar larvae per l

10 third instar larvae per l and

5 fourth instar larvae per l.

In the embodiment illustrated in FIG. 7, there is provided asemi-recirculation system of alternative construction to that of FIG. 1in that an annular larval rearing tank 50 comprises an assembly ofmodular straight channel sections 51 and corner channel sections 52. Thechannel sections 51 and 52 are formed of substantially opaque plasticsmaterial and have a wall height and width across the channel of 30 cmeach. The moulded sections are adapted to be bolted together to form thetank 50 and sealed with silicon sealant.

Two sub-tanks 53, 54 are provided, each having a capacity the same ormore than the larval rearing tank 50. A UV sterilizer unit 55 andsubmersible pump 56 circulates medium that has been filtered to 0.5-1.0μm and pre-sterilized by 10% chlorine for a period of 12 hours, and thenneutralised with 10% sodium thiosulphate. The new rearing water istested by Palintest® (DPD No 1) before introducing into the rearingsystem to make sure no chlorine remains.

The medium is delivered through the UV sterilizer 55 and subjected to UVradiation at about 10 l/hour/Watt. The sterilizer 55 incorporatestemperature control means comprising a heater/chiller/thermostat tomaintain the temperature within ±0.5° C. Medium passes to the tank 50 bydelivery pipe 57 which is manifolded via a splitter pipe 60 to inner 61and outer 62 ring mains located at the upper edge of the inner and outerwalls of the tank 50.

A plurality of droppers 63 extend down the respective walls from thering mains 61 and 62 toward the bottom of the tank 50, each dropperterminating in a nozzle 64 oriented in the direction of circulation andaway from the respective walls.

A 1 mm² meshed drain 65 enables recirculation to the subtank in use andlevel control in the tank 50.

Basic operation of the system is that water in one sub-tank iscirculated into the larval rearing tank for 24 hours, using thesubmersible pump. After 24 hours, the pump is transferred to the othersub-tank, with the water controlled at the same temperature (±0.5° C.).Water is then circulated into the larval rearing tank again. While waterin one sub-tank is being used, the other sub-tank is emptied to wasteand dried in preparation for recharging with pretreated medium.

In the embodiment of FIG. 8, this is substantially as per that of FIG.7, except that a one way flow through system adapted to use sea watersupply is illustrated. This system delivers seawater that has beenfiltered down to at least 1.0 μm and preferably 0.5 μm to the UVsterilizer/temperature control assembly 55 and thence to the deliverypipe. The drain 65 takes the flow passing through the system directly towaste.

In the embodiment of FIG. 9 there is illustrated a fully recirculatingsystem with medium regeneration. In this embodiment there is illustrateda plurality of the tanks 50 supplied with medium via a multiple splitterpipe 60. A reservoir 66 holding the same volume as the sum of the tankcapacities is provided. The reservoir has a sump portion 67 adapted toreceive medium from the drains 65 which are manifolded to the sump 67. Apump 70 delivers the medium to a biofilter 71, foam fractionator 72 anda processing unit 73 which integrates the functions of a UV sterilizer,ozone generator and mixer power head with venturi, as well astemperature control.

Food for larvae

To obtain a standard quality of food, enrichment of bivalves isnecessary. The green micro-algae Nannochloropsis spp. is cocultured withlive pipis at a temperature range maintained between 25-28° C. The celldensity is maintained at more than 20×10⁷ per ml. The algal culture isused at a rate of 1 kg of pipis wet weight per 40 l of algae water, andreplace the water every 12 hours. The level of ammonia in the algaewater is maintained below 1 ppm. The enrichment process is continued for48 hours. The pipis yield 20% pipi flesh based on wet weight of pipis inthe shell.

The enriched pipis were chopped roughly with a curved “dick knife” on acutting board. The pieces were washed through a large mesh of 0.5 to 2.0mm depending on the larval stage to be fed, and then a small mesh of 0.5mm. the large mesh used was 1.0 mm for the first instar, 1.5 mm for the2nd instar and 2.0 m for the 3rd and 4th instars. The pieces of choppedflesh retained between the large and small mesh sizes were set aside.The pieces of flesh retained in the large size mesh were chopped again,repeating the above process.

The processed food was sterilised before feeding by washing the flesh inUV sterilised seawater carefully, and then incubating in 0.1% chlorineseawater solution for a period of 30 minutes. The washed food particleswere washed by UV sterilised seawater again on the small mesh beforefeeding to larvae.

The prepared food materials with sterilised seawater were distributedequally in the rearing water using a pipet. Food particles sink to thebottom of the rearing tank. Food particles remaining in the rearing tankafter feeding was cleaned out before adding the next lot of food.

The feeding level changes depending on growth stage and intermoultstage. The level of feeding was adjusted by taking note of how much foodremains from the previous feed. The following is the daily feed levelnormalised for 1000 phyllosomas in a 1 ton tank.

First instar (Day 1-Day 5)

(Day 1)

Phyllosomas started eating from the night of hatching (50 ml of choppedflesh particles). To obtain synchrony of larval moult, there was nofeeding on the morning of Day 1.

(Day 2-4)

Phyllosomas started eating more, so the feeding level was adjusteddepending on level of remaining food. Feeding was twice a day (50-70 mlin the early morning and late evening).

(Day 5-6)

Phyllosomas started preparing to moult, so the feeding level wasdecreased from the evening of Day 5 (50-70 ml in the morning, and 30-40ml in the late afternoon). Second instar (Day 6-Day 10)

(Day 6-7)

First instar phyllosomas moulted to the second instar in the earlymorning, so the feeding level in the morning was minimised, with more inthe late evening (50-60 ml in the morning, and 60-80 ml in theafternoon).

(Day 7-9)

Feeding twice a day was used (50-60 ml), but towards Day 9 phyllosomasstarted to eat more, and feeding 3 times a day became necessary.

(Day 9-10)

Feeding levels were still be high, even before mounting. Enough food wasmade available through the nights (70-80 ml) to avoid cannibalism in themorning.

Third instar (Day 10-16)

(Day 10-12)

Larvae moulted in the early morning (4-5 am), and therefore it was madesure enough food is available before and during the moulting stage. Anextra feeding (20-30 ml) before moulting was made, as there was no foodremaining in the tank. Post-moult stage larvae did not eat food for 3-6hours, and therefore the morning feed was minimised (50-60 ml), with ahigher level in the afternoon (100 ml-120 ml).

(Day 12-16)

Larvae were fed 3 times a day (70-80 ml, every 8 hours), making surethat food was always available.

Fourth instar (Day 15-27)

(Day 15-17)

Fed three times a day (60-70 ml), making sure that food was alwaysavailable.

(Day 18-21)

The feeding level of phyllosomas was now at its peak. Larvae were fedthree times a day (100-120 ml).

(Day 21-30)

Phyllosomas started to metamorphose to the nisto stage, and thereforethe feeding level was decreased with the decreasing number of fourthinstar phyllosomas. When phyllosomas are not eating food between days25-26), feeding was reduced to only twice a day (60-80 ml). To avoidcannibalism, separation of pre-metamorphosis phyllosomas from otherintermoult phyllosomas was performed.

NISTO

The nistos are reared in the same design tank as per larvae, with thewater treated in the same manner.

After metamorphosis to the nisto stage, nistos are reared in a nistotank. Water quality in the nisto rearing tank is the same as that ofphyllosoma rearing. Nistos can be reared at a high density (>100 nistosper l). During the nisto stage, feeding is not required. The duration ofthe nisto stage is approximately 7 days, with the temperature kept at26-27° C.

JUVENILE CULTURE

After 7 days, nistos moult to the juvenile stage. Moulting to juvenilesalways occurs during the night. The exoskeletons of juveniles arecalcified and pigmented. Newly moulted juveniles should be collectedfrom the nisto tank the next morning and transferred to the juvenileholding tank.

Juveniles are nocturnal. Feed only once a day in the evening and cleanthe remaining food and faeces out the next morning. Adjust feedinglevels according to the amount of remaining food. The food size forfirst instar juveniles is similar size to that of second or third instarphyllosomas. Chopped flesh of enriched pipis is suitable for up to atleast fourth instar juveniles, then non-enriched pipis, squids, scallopsand mussels can be used. The optimal temperature for culturing juvenilesis 26-27° C.

It will of course be realised that while the foregoing has been given byway of illustrative example of this invention, all such and othermodifications and variations thereto as would be apparent to personsskilled in the art are deemed to fall within the broad scope and ambitof this invention as defined in the claims appended hereto.

I claim:
 1. A Thenus spp., rock lobster or slipper lobster larva raisingmethod including the steps of: providing an annular tank adapted to holdlarva raising medium to a depth of at least 10 cm to less than 1 meter;continuously supplying substantially sterilized, filtered larva raisingmedium to said tank through a plurality of outlets disposed about anannular side wall of the tank and adapted to cause horizontalcirculation of said medium and having an outlet flow velocity selectedto prevent larva damage; continuously draining said medium through atank depth-regulating drain assembly including a larva screen having aflow velocity of said medium therethrough selected to prevent damage tolarvae, and maintaining said medium at a temperature selected toaccommodate the larva species to be raised by control of the temperatureof said substantially sterilized, filtered larva raising medium.
 2. Amethod according to claim 1, wherein said temperature is controlled tobe at substantially the same temperature ±0.5° C. as the source of thelarva introduced to said tank.
 3. A method according to claim 2, whereinthe salinity of said medium is maintained in the range of between 25-40ppt.
 4. A method according to claim 2, wherein said circulation andstocking density is selected whereby the oxygen level of the rearingwater is maintained at a level of at least 7 ppm, at 26-27° C.
 5. Amethod according to claim 1, wherein the larva are phyllosomas ofMoreton bay bugs, and wherein the temperature range of the medium ismaintained between 26-27° C.
 6. A method according to claim 5, whereinthe salinity of said medium is maintained in the range of between 25-40ppt.
 7. A method according to claim 6, wherein said salinity is variedby less than 1 ppt per day.
 8. A method according to claim 6, whereinthe level of pH is kept at between 7-9.
 9. A method according to claim5, wherein the level of pH is kept at between 7-9.
 10. A methodaccording to claim 5, wherein said circulation and stocking density isselected whereby the oxygen level of the rearing water is maintained ata level of at least 7 ppm, at 26-27° C.
 11. A method according to claim10, wherein the maximum rearing densities of phyllosomas are: 40 firstinstar larvae per l 25 second instar larvae per l 10 third instar larvaeper l and 5 fourth instar larvae per l.
 12. A method according to claim5, wherein said temperature is controlled to be at substantially thesame temperature ±0.5° C. as the source of the larva introduced to saidtank.
 13. Thenus spp., rock lobster or slipper lobster larva raisingapparatus including: a supply of substantially sterilized, filteredlarva raising medium; an annular tank adapted to hold larva raisingmedium to a depth of at least 10 cm to less than 1 meter; a plurality ofoutlets on an annular side wall of the tank, connected to said supplyand adapted to deliver and cause horizontal circulation of said mediumin said tank; drain means including a larva screen having a flowvelocity of said medium therethrough selected to prevent damage tolarvae and configured to maintain a selected level in said tank, andtemperature control means for said medium supply.
 14. Crustacean larvaraising apparatus according to claim 13, wherein said larval rearingtank comprises an annular raceway having straight portions closed bysubstantially part-circular end portions.
 15. Crustacean larva raisingapparatus according to claim 14, wherein said raceway comprises amodular construction of said substantially part-circular end andstraight portions, whereby the linear dimensions and thus holdingcapacity may be selected.
 16. Crustacean larva raising apparatusaccording to claim 15, wherein said corner and straight portions aremoulded in plastics material and are adapted to be bolted up in assemblyto form the raceway.
 17. Crustacean larva raising apparatus according toclaim 14, wherein said end and straight portions are moulded in plasticsmaterial and are adapted to be bolted up in assembly to form theraceway.
 18. Crustacean larva raising apparatus according to claim 17,wherein said tank has a depth of less than one meter.
 19. Crustaceanlarva raising apparatus according to claim 13, wherein said tank has adepth of less than one meter.
 20. Crustacean larva raising apparatusaccording to claim 19, wherein the medium depth is maintained between 10to 20 cm.
 21. Crustacean larva raising apparatus according to claim 13,wherein the medium depth is maintained between 10 to 20 cm. 22.Crustacean larva raising apparatus according to claim 21, wherein saidtank is adapted to be arrayed in stacks.
 23. Crustacean larva raisingapparatus according to claim 13, wherein said tank is adapted to bearrayed in stacks.
 24. Crustacean larva raising apparatus according toclaim 23, wherein said water outlet comprise a plurality of nozzlesdirected to encourage the continuous one way circulation with consistentflow about the circuit.
 25. Crustacean larva raising apparatus accordingto claim 13, wherein said water outlets comprise a plurality of nozzlesdirected to encourage the continuous one way circulation with consistentflow about the circuit.
 26. Crustacean larva raising apparatus accordingto claim 25, wherein said nozzles in use have a flow velocity of lessthan 6 m per minute.
 27. Crustacean larva raising apparatus according toclaim 26, wherein said flow velocity is the minimum flow rate consistentwith maintaining circulation of the medium in the tank.
 28. Crustaceanlarva raising apparatus according to claim 27, wherein said plurality ofoutlets are manifolded to said continuous supply by a linear or ringmains manifold.
 29. Crustacean larva raising apparatus according toclaim 13, wherein said plurality of outlets are manifolded to saidcontinuous supply by a linear or ring mains manifold.
 30. Crustaceanlarva raising apparatus according to claim 29, wherein said linear orring mains manifold is disposed about an upper portion of the tank abovethe selected level of said medium, said outlets being in the region ofthe bottom of the tank and connected to said manifold by droppers. 31.Crustacean larva raising apparatus according to claim 30, wherein saidtank is an annular tank and wherein one said manifolds disposed on eachof the upper portions of the inner and outer walls of the tank. 32.Crustacean larva raising apparatus according to claim 31, wherein saidplurality of outlets comprise nozzles and said horizontal circulation iseffected by aiming said nozzles in a direction having a component in thedirection of desired circulation and a component in a direction inwardlyof a side wall of said tank.
 33. Crustacean larva raising apparatusaccording to claim 13, wherein said plurality of outlets comprisenozzles and said horizontal circulation is effected by aiming saidnozzles in a direction having a component in the direction of desiredcirculation and a component in a direction inwardly of a side wall ofsaid tank.
 34. Crustacean larva raising apparatus according to claim 33,wherein said medium supply is a continuous supply selected fromrecirculating non-recirculating and partial recirculating supplies. 35.Crustacean larva raising apparatus according to claim 13, wherein saidmedium supply is a continuous supply selected from recirculating,non-recirculating and partial recirculating supplies.
 36. Crustaceanlarva raising apparatus according to claim 35, wherein said medium isfiltered to a particle size of less than 1 mm.
 37. Crustacean larvaraising apparatus according to claim 13, wherein said medium is filteredto a particle size of less than 1 mm.
 38. Crustacean larva raisingapparatus according to claim 37, wherein said medium is sterilized byone or more of physical, chemical or radiation means.
 39. Crustaceanlarva raising apparatus according to claim 13, wherein said medium issterilized by one or more of physical, chemical or radiation means. 40.Crustacean larva raising apparatus according to claim 39, wherein saidsterilization in by one or more of UV sterilization, submicronfiltration, chlorination/neutralisation, acidification/neutralisationand ozonisation.
 41. Crustacean larva raising apparatus according toclaim 40, wherein said drain means is provided with a meshed drainopening having a mesh size of about 1 mm.
 42. Crustacean larva raisingapparatus according to claim 13, wherein said drain means is providedwith a meshed drain opening having a mesh size of about 1 mm. 43.Crustacean larva raising apparatus according to claim 42, wherein saidmeshed drain opening is selected to have an outflow velocity selected tobe less that the outlet flow velocity through the outlets in use. 44.Crustacean larva raising apparatus according to claim 43, wherein saiddrain means comprises a surface drain set to a medium level of from 10to 20 cm.
 45. Crustacean larva raising apparatus according to claim 42,wherein said drain means comprises a surface drain set to a medium levelof from 10 to 20 cm.
 46. Crustacean larva raising apparatus according toclaim 45, wherein said surface drain is set on a standpipe which isadjustable in length.
 47. Crustacean larva raising apparatus accordingto claim 46, wherein there is further provided cover means for saidtank, the cover means and said tank being selected to be substantiallyopaque to ambient light in use.
 48. Crustacean larva raising apparatusaccording to claim 13, wherein said drain means comprises a surfacedrain set to a medium level of from said tank being selected to besubstantially opaque to ambient light in use.
 49. Crustacean larvaraising apparatus according to claim 48, wherein said temperaturecontrol means includes a thermostat and comprises one or both of aheater and chiller.
 50. Crustacean larva raising apparatus according toclaim 13, wherein said temperature control means includes a thermostatand comprises one or both of a heater and chiller.
 51. Crustacean larvaraising apparatus according to claim 50, wherein said temperaturecontrol means is selected to maintain the temperature of said medium toa selected temperature ±0.5° C.